Internet DRAFT - draft-ietf-avtcore-rfc5285-bis
draft-ietf-avtcore-rfc5285-bis
AVTCore D. Singer
Internet-Draft Apple, Inc.
Obsoletes: 5285 (if approved) H. Desineni
Intended status: Standards Track Qualcomm
Expires: February 3, 2018 R. Even, Ed.
Huawei Technologies
August 2, 2017
A General Mechanism for RTP Header Extensions
draft-ietf-avtcore-rfc5285-bis-14.txt
Abstract
This document provides a general mechanism to use the header
extension feature of RTP (the Real-Time Transport Protocol). It
provides the option to use a small number of small extensions in each
RTP packet, where the universe of possible extensions is large and
registration is de-centralized. The actual extensions in use in a
session are signaled in the setup information for that session. This
document obsoletes RFC5285.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 3, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1.1. Transmission Considerations . . . . . . . . . . . . . 5
4.1.2. Header Extension Type Considerations . . . . . . . . 6
4.2. One-Byte Header . . . . . . . . . . . . . . . . . . . . . 7
4.3. Two-Byte Header . . . . . . . . . . . . . . . . . . . . . 9
5. SDP Signaling Design . . . . . . . . . . . . . . . . . . . . 10
6. SDP Signaling for support of mixed one byte and two bytes
header extensions. . . . . . . . . . . . . . . . . . . . . . 12
7. SDP Offer/Answer . . . . . . . . . . . . . . . . . . . . . . 13
8. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10.1. Identifier Space for IANA to Manage . . . . . . . . . . 17
10.2. Registration of the SDP extmap Attribute . . . . . . . . 19
10.3. Registration of the SDP extmap-allow-mixed Attribute . . 19
11. Changes from RFC5285 . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The RTP specification [RFC3550] provides a capability to extend the
RTP header. It defines the header extension format and rules for its
use in Section 5.3.1. The existing header extension method permits
at most one extension per RTP packet, identified by a 16-bit
identifier and a 16-bit length field specifying the length of the
header extension in 32-bit words.
This mechanism has two conspicuous drawbacks. First, it permits only
one header extension in a single RTP packet. Second, the
specification gives no guidance as to how the 16-bit header extension
identifiers are allocated to avoid collisions.
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This specification removes the first drawback by defining a backward-
compatible and extensible means to carry multiple header extension
elements in a single RTP packet. It removes the second drawback by
defining that these extension elements are named by URIs, defining an
IANA registry for extension elements defined in IETF specifications,
and a Session Description Protocol (SDP) method for mapping between
the naming URIs and the identifier values carried in the RTP packets.
This header extension applies to RTP/AVP (the Audio/Visual Profile)
and its extensions.
This document obsoletes [RFC5285] and removes a limitation from
RFC5285 that did not allow sending both one-byte and two-byte header
extensions in the same RTP stream.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Design Goals
The goal of this design is to provide a simple mechanism whereby
multiple identified extensions can be used in RTP packets, without
the need for formal registration of those extensions but nonetheless
avoiding collision.
This mechanism provides an alternative to the practice of burying
associated metadata into the media format bit stream. This has often
been done in media data sent over fixed-bandwidth channels. Once
this is done, a decoder for the specific media format needs to
extract the metadata. Also, depending on the media format, the
metadata can be added at the time of encoding the media so that the
bit-rate used for the metadata is taken into account. But the
metadata can be unknown at that time. Inserting metadata at a later
time can cause a decode and re-encode to meet bit-rate requirements.
In some cases, a more appropriate, higher-level mechanism may be
available, and if so, it can be used. For cases where a higher-level
mechanism is not available, it is better to provide a mechanism at
the RTP level than have the metadata be tied to a specific form of
media data.
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4. Packet Design
4.1. General
The following design is fit into the "header extension" of the RTP
extension, as described above.
The presence and format of this header extension and its contents are
negotiated or defined out-of-band, such as through signaling (see
below for SDP signaling). The 16-bit identifier for the two forms of
RTP extension defined here is only an architectural constant (e.g.,
for use by network analyzers); it is the negotiation/definition
(e.g., in SDP) that is the definitive indication that this header
extension is present.
The RTP specification [RFC3550] states that RTP "is designed so that
the header extension may be ignored by other interoperating
implementations that have not been extended". The intent of this
restriction is that RTP header extensions MUST NOT be used to extend
RTP itself in a manner that is backwards incompatible with non-
extended implementations. For example, a header extension is not
allowed to change the meaning or interpretation of the standard RTP
header fields, or of the RTCP Control Protocol (RTCP). Header
extensions MAY carry metadata in addition to the usual RTP header
information, provided the RTP layer can function if that metadata is
missing. For example, RTP header extensions can be used to carry
data that's also sent in RTCP, as an optimisation to lower latency,
since they'll fall back to the original, non-optimised, behaviour if
the header extension is not present. The use of header extensions to
convey information that will, if missing, disrupt the behaviour of a
higher layer application that builds on top of RTP is only acceptable
if this doesn't affect interoperability at the RTP layer. For
example, applications that use the SDP BUNDLE extension with the MID
RTP header extension [I-D.ietf-mmusic-sdp-bundle-negotiation] to
correlate RTP streams with SDP m= lines likely won't work with full
functionality if the MID is missing, but the operation of the RTP
layer of those applications will be unaffected. Support for RTP
header extensions based on this memo is negotiated using, for
example, SDP Offer/Answer [RFC3264]; intermediaries aware of the RTP
header extensions are advised to be cautious when removing or
generating RTP header extensions see section 4.7 of [RFC7667].
The RTP header extension is formed as a sequence of extension
elements, with possible padding. Each extension element has a local
identifier and a length. The local identifiers MAY be mapped to a
larger namespace in the negotiation (e.g., session signaling).
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4.1.1. Transmission Considerations
As is good network practice, data should only be transmitted when
needed. The RTP header extension SHOULD only be present in a packet
if that packet also contains one or more extension elements, as
defined here. An extension element SHOULD only be present in a
packet when needed; the signaling setup of extension elements
indicates only that those elements can be present in some packets,
not that they are in fact present in all (or indeed, any) packets.
Some general considerations for getting the header extensions
delivered to the receiver:
1. The probability for packet loss and burst loss determine how many
repetitions of the header extensions will be required to reach a
targeted delivery probability, and if burst loss is likely, what
distribution would be needed to avoid getting all repetitions of
the header extensions lost in a single burst.
2. If a set of packets are all needed to enable decoding, there is
commonly no reason for including the header extension in all of
these packets, as they share fate. Instead, at most one instance
of the header extension per independently decodable set of media
data would be a more efficient use of the bandwidth.
3. How early the Header Extension item information is needed, from
the first received RTP data or only after some set of packets are
received, can guide if the header extension(s) should be in all
of the first N packets or be included only once per set of
packets, for example once per video frame.
4. The use of RTP level robustness mechanisms, such as RTP
retransmission [RFC4588], or Forward Error Correction, e.g.,
[RFC5109] may treat packets differently from a robustness
perspective, and header extensions should be added to packets
that get a treatment corresponding to the relative importance of
receiving the information.
As a summary, the number of header extension transmissions should be
tailored to a desired probability of delivery taking the receiver
population size into account. For the very basic case, N repetitions
of the header extensions should be sufficient, but may not be
optimal. N is selected so that the header extension target delivery
probability reaches 1-P^N, where P is the probability of packet loss.
For point to point or small receiver populations, it might also be
possible to use feedback, such as RTCP, to determine when the
information in the header extensions has reached all receivers and
stop further repetitions. Feedback that can be used includes the
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RTCP XR Loss RLE report block [RFC3611], which will indicate
successful delivery of particular packets. If the RTP/AVPF Transport
Layer Feedback Messages for generic NACK [RFC4585] is used, it can
indicate the failure to deliver an RTP packet with the header
extension, thus indicating the need for further repetitions. The
normal RTCP report blocks can also provide an indicator of successful
delivery, if no losses are indicated for a reporting interval
covering the RTP packets with the header extension. Note that loss
of an RTCP packet reporting on an interval where RTP header extension
packets were sent, does not necessarily mean that the RTP header
extension packets themselves were lost.
4.1.2. Header Extension Type Considerations
Each extension element in a packet has a local identifier (ID) and a
length. The local identifiers present in the stream MUST have been
negotiated or defined out-of-band. There are no static allocations
of local identifiers. Each distinct extension MUST have a unique ID.
The ID value 0 is reserved for padding and MUST NOT be used as a
local identifier.
An extension element with an ID value equal 0 MUST NOT have len field
greater than 0. If such an extension element is encountered, its
length field MUST be ignored, processing of the entire extension MUST
terminate at that point, and only the extension elements present
prior to the element with ID 0 and len field greater than 0 SHOULD be
considered.
There are two variants of the extension: one-byte and two-byte
headers. Since it is expected that (a) the number of extensions in
any given RTP session is small and (b) the extensions themselves are
small, the one-byte header form is preferred and MUST be supported by
all receivers. A stream MUST contain only one-byte or only two-byte
headers unless it is known that all recipients support mixing, either
by SDP Offer/Answer [RFC3264] negotiation (see section 6) or by out-
of-band knowledge. Each RTP packet with an RTP header extension
following this specification will indicate if it contains one or two
byte header extensions through the use of the "defined by profile"
field. Extension element types that do not match the header
extension format, i.e. one- or two-byte, MUST NOT be used in that RTP
packet. Transmitters SHOULD NOT use the two-byte form when all
extensions are small enough for the one-byte header form.
Transmitters that intend to send the two-byte form SHOULD negotiate
the use of IDs above 14 if they want to let the Receivers know that
they intend to use two-byte form, for example if the RTP header
extension is longer than 16 bytes. A transmitter may be aware that
an intermediary may add RTP header extensions; in this case the
transmitter SHOULD use two-byte form.
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A sequence of extension elements, possibly with padding, forms the
header extension defined in the RTP specification. There are as many
extension elements as fit into the length as indicated in the RTP
header extension length. Since this length is signaled in full
32-bit words, padding bytes are used to pad to a 32-bit boundary.
The entire extension is parsed byte-by-byte to find each extension
element (no alignment is needed), and parsing stops at the earlier of
the end of the entire header extension, or in "one-byte headers only"
case, on encountering an identifier with the reserved value of 15.
In both forms, padding bytes have the value of 0 (zero). They MAY be
placed between extension elements, if desired for alignment, or after
the last extension element, if needed for padding. A padding byte
does not supply the ID of an element, nor the length field. When a
padding byte is found, it is ignored and the parser moves on to
interpreting the next byte.
Note carefully that the one-byte header form allows for data lengths
between 1 and 16 bytes, by adding 1 to the signaled length value
(thus, 0 in the length field indicates 1 byte of data follows). This
allows for the important case of 16-byte payloads. This addition is
not performed for the two-byte headers, where the length field
signals data lengths between 0 and 255 bytes.
Use of RTP header extensions will reduce the efficiency of RTP header
compression, since the header extension will be sent uncompressed
unless the RTP header compression module is updated to recognize the
extension header. If header extensions are present in some packets,
but not in others, this can also reduce compression efficiency by
requiring an update to the fixed header to be conveyed when header
extensions start or stop being sent. The interactions of the RTP
header extension and header compression is explored further in
[RFC2508] and [RFC3095].
4.2. One-Byte Header
In the one-byte header form of extensions, the 16-bit value required
by the RTP specification for a header extension, labeled in the RTP
specification as "defined by profile", MUST have the fixed bit
pattern 0xBEDE (the pattern was picked for the trivial reason that
the first version of this specification was written on May 25th the
feast day of the Venerable Bede).
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Each extension element MUST start with a byte containing an ID and a
length:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| ID | len |
+-+-+-+-+-+-+-+-+
The 4-bit ID is the local identifier of this element in the range
1-14 inclusive. In the signaling section, this is referred to as the
valid range.
The local identifier value 15 is reserved for future extension and
MUST NOT be used as an identifier. If the ID value 15 is
encountered, its length field MUST be ignored, processing of the
entire extension MUST terminate at that point, and only the extension
elements present prior to the element with ID 15 SHOULD be
considered.
The 4-bit length is the number minus one of data bytes of this header
extension element following the one-byte header. Therefore, the
value zero in this field indicates that one byte of data follows, and
a value of 15 (the maximum) indicates element data of 16 bytes.
(This permits carriage of 16-byte values, which is a common length of
labels and identifiers, while losing the possibility of zero-length
values -- which would often be padded anyway.)
An example header extension, with three extension elements, and some
padding follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xBE | 0xDE | length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | L=0 | data | ID | L=1 | data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...data | 0 (pad) | 0 (pad) | ID | L=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4.3. Two-Byte Header
In the two-byte header form, the 16-bit value defined by the RTP
specification for a header extension, labeled in the RTP
specification as "defined by profile", is defined as shown below.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x100 |appbits|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The appbits field is 4 bits that are application-dependent and MAY be
defined to be any value or meaning, and are outside the scope of this
specification. For the purposes of signaling, this field is treated
as a special extension value assigned to the local identifier 256.
If no extension has been specified through configuration or signaling
for this local identifier value 256, the appbits field SHOULD be set
to all 0s by the sender and MUST be ignored by the receiver.
Each extension element starts with a byte containing an ID and a byte
containing a length:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 8-bit ID is the local identifier of this element in the range
1-255 inclusive. In the signaling section, the range 1-256 is
referred to as the valid range, with the values 1-255 referring to
extension elements, and the value 256 referring to the 4-bit field
'appbits' (above). Note that there is one ID space for both one-byte
and two-byte form. This means that the lower values (1-14) can be
used in the 4-bit ID field in the one-byte header format with the
same meanings.
The 8-bit length field is the length of extension data in bytes not
including the ID and length fields. The value zero indicates there
is no data following.
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An example header extension, with three extension elements, and some
padding follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x10 | 0x00 | length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | L=0 | ID | L=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data | 0 (pad) | ID | L=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5. SDP Signaling Design
The indication of the presence of this extension, and the mapping of
local identifiers used in the header extension to a larger namespace,
MUST be performed out-of-band, for example, as part of an SDP Offer/
Answer [RFC3264]. This section defines such signaling in SDP.
A usable mapping MUST use IDs in the valid range, and each ID in this
range MUST be used only once for each media (or only once if the
mappings are session level). Mappings that do not conform to these
rules MAY be presented, for instance, during SDP Offer/Answer
[RFC3264] negotiation as described in the next section, but remapping
to conformant values is necessary before they can be applied.
Each extension is named by a URI. That URI MUST be absolute, and
precisely identifies the format and meaning of the extension. URIs
that contain a domain name SHOULD also contain a month-date in the
form mmyyyy. The definition of the element and assignment of the URI
MUST have been authorized by the owner of the domain name on or very
close to that date. (This avoids problems when domain names change
ownership.) If the resource or document defines several extensions,
then the URI MUST identify the actual extension in use, e.g., using a
fragment or query identifier (characters after a '#' or '?' in the
URI).
Rationale: the use of URIs provides for a large, unallocated space,
and gives documentation on the extension. The URIs do not have to be
de-referencable, in order to permit confidential or experimental use,
and to cover the case when extensions continue to be used after the
organization that defined them ceases to exist.
An extension URI with the same attributes MUST NOT appear more than
once applying to the same stream, i.e., at session level or in the
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declarations for a single stream at media level. (The same extension
can, of course, be used for several streams, and can appear with
different extensionattributes for the same stream.)
For extensions defined in RFCs, the URI used SHOULD be a URN starting
"urn:ietf:params:rtp-hdrext:" and followed by a registered,
descriptive name.
The registration requirements are detailed in the IANA Considerations
section, below.
An example (this is only an example), where 'avt-example-metadata' is
the hypothetical name of a header extension, might be:
urn:ietf:params:rtp-hdrext:avt-example-metadata
An example name not from the IETF (this is only an example) might be:
http://example.com/082005/ext.htm#example-metadata
The mapping MAY be provided per media stream (in the media-level
section(s) of SDP, i.e., after an "m=" line) or globally for all
streams (i.e., before the first "m=" line, at session level). The
definitions MUST be either all session level or all media level; it
is not permitted to mix the two styles. In addition, as noted above,
the IDs used MUST be unique in each media section of the SDP, or
unique in the session for session-level SDP declarations.
Each local identifier potentially used in the stream is mapped to an
extension identified by a URI using an attribute of the form:
a=extmap:<value>["/"<direction>] <URI> <extensionattributes>
where <URI> is a URI, as above, <value> is the local identifier (ID)
of this extension and is an integer in the valid range (0 is reserved
for padding in both forms, and 15 is reserved in the one-byte header
form, as noted above), and <direction> is one of "sendonly",
"recvonly", "sendrecv", or "inactive" (without the quotes) with
relation to the device being configured.
The formal BNF syntax is presented in a later section of this
specification.
Example:
a=extmap:1 http://example.com/082005/ext.htm#ttime
a=extmap:2/sendrecv http://example.com/082005/ext.htm#xmeta short
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When SDP signaling is used for the RTP session, it is the presence of
the 'extmap' attribute(s) that is diagnostic that this style of
header extensions is used, not the magic number ("BEDE" or "100")
indicated above.
6. SDP Signaling for support of mixed one byte and two bytes header
extensions.
In order to allow for backward interoperability with systems that do
not support mixing of one byte and two bytes header extensions this
document defines the "a=extmap-allow-mixed" Session Description
Protocol (SDP) [RFC4566] attribute to indicate if the participant is
capable of supporting this new mode. The attribute takes no value.
This attribute can be used at the session or media levels. A
participant that proposes the use of this mode SHALL itself support
the reception of mixed one byte and two bytes header extensions.
If SDP Offer/Answer [RFC3264] is supported and used,the negotiation
for mixed one byte and two bytes extension MUST be negotiated using
SDP Offer/Answer [RFC3264]. In the absence of negotiations using SDP
Offer/Answer, for example when declarative SDP is used, mixed headers
MUST NOT occur unless the transmitter has some (out of band)
knowledge that all potential recipients support this mode.
The formal definition of this attribute is:
Name: extmap-allow-mixed
Value: none
Usage Level: session, media
Charset Dependent: no
Example:
a=extmap-allow-mixed
When doing SDP Offer/Answer [RFC3264] an offering client that wishes
to use both one and two bytes extensions MUST include the attribute
"a= extmap-allow-mixed " in the SDP offer. If "a= extmap-allow-mixed
" is present in the offer SDP, the answerer that supports this mode
and wishes to use it SHALL include the "a=extmap-allow-mixed "
attribute in the answer. In the cases where the attribute has been
excluded, both clients SHALL NOT use mixed one bytes and two bytes
extensions in the same RTP stream but MAY use one-byte or two-bytes
form exclusively (see section 4.1.2).
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When used in [I-D.ietf-mmusic-sdp-bundle-negotiation] this attribute
is specified as identical category for the
[I-D.ietf-mmusic-sdp-mux-attributes]. This allows for only a subset
of the m-lines in the bundle group to offer extmap-allow-mixed. When
an answerer supporting the extmap-allow-mix attribute receives an
offer where only some of the m-lines in the bundle group include the
extmap-allow-mixed attribute, the answerer MUST receive this offer
and support mixed one-byte and two-bytes only for those m-lines.
Transmitters MUST only send RTP header extensions using mixed on
those RTP streams originating from those media sources (m=) blocks
that includes extmap-allow-mixed, and are RECOMMENDED to support
receiving mixed on all RTP streams being received in an RTP session
where at least one bundled m= block is indicating extmap-allow-mixed.
7. SDP Offer/Answer
The simple signaling described above for the extmap attribute MAY be
enhanced in an SDP Offer/Answer [RFC3264] context, to permit:
o asymmetric behavior (extensions sent in only one direction),
o the offer of mutually exclusive alternatives, or
o the offer of more extensions than can be sent in a single session.
A direction attribute MAY be included in an extmap; without it, the
direction implicitly inherits, of course, from the stream direction,
or is "sendrecv" for session-level attributes or extensions of
"inactive" streams. The direction MUST be one of "sendonly",
"recvonly", "sendrecv", or "inactive" as specified in [RFC3264]
Extensions, with their directions, MAY be signaled for an "inactive"
stream. It is an error to use an extension direction incompatible
with the stream direction (e.g., a "sendonly" attribute for a
"recvonly" stream).
If an offer or answer contains session-level mappings (and hence no
media-level mappings), and different behavior is desired for each
stream, then the entire set of extension map declarations MAY be
moved into the media-level section(s) of the SDP. (Note that this
specification does not permit mixing global and local declarations,
to make identifier management easier.)
If an extension map is offered as "sendrecv", explicitly or
implicitly, and asymmetric behavior is desired, the SDP answer MAY be
changed to modify or add direction qualifiers for that extension.
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If an extension is marked as "sendonly" and the answerer desires to
receive it, the extension MUST be marked as "recvonly" in the SDP
answer. An answerer that has no desire to receive the extension or
does not understand the extension SHOULD remove it from the SDP
answer. An answerer MAY want to respond that he supports the
extension and does not want to receive it at the moment but may offer
to receive it in a future offer, will mark the extension as
"inactive"
If an extension is marked as "recvonly" and the answerer desires to
send it, the extension MUST be marked as "sendonly" in the SDP
answer. An answerer that has no desire to, or is unable to, send the
extension SHOULD remove it from the SDP answer. An answerer MAY want
to respond that he support this extension yet has no intention of
sending it now but may offer to send it in a future offer by marking
the extension as "inactive"
Local identifiers in the valid range inclusive in an offer or answer
must not be used more than once per media section (including the
session-level section). The local identifiers MUST be unique in an
RTP session and the same identifier MUST be used for the same offered
extension in the answer. A session update MAY change the direction
qualifiers of extensions under use. A session update MAY add or
remove extension(s). Identifiers values in the valid range MUST NOT
be altered (remapped).
Note that, under this rule, the same local identifier cannot be used
for two extensions for the same media, even when one is "sendonly"
and the other "recvonly", as it would then be impossible to make
either of them sendrecv (since re-numbering is not permitted either).
If a party wishes to offer mutually exclusive alternatives, then
multiple extensions with the same identifier in the extended range
4096-4351 MAY be offered; the answerer SHOULD select at most one of
the offered extensions with the same identifier, and remap it to a
free identifier in the valid range, for that extension to be usable.
Similarly, if more extensions are offered than can be fit in the
valid range, identifiers in the range 4096-4351 MAY be offered; the
answerer SHOULD choose those that are desired, and remap them to a
free identifier in the valid range.
An answerer may copy an extmap for an identifier in the extended
range into the answer to indicate to the offerer that it supports
that extension. Of course, such an extension cannot be used, since
there is no way to specify them in an extension header. If needed,
the offerer or answerer can update the session to assign a valid
identifier to that extension URI.
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Rationale: the range 4096-4351 for these negotiation identifiers is
deliberately restricted to allow expansion of the range of valid
identifiers in future.
Either party MAY include extensions in the stream other than those
negotiated, or those negotiated as "inactive", for example, for the
benefit of intermediate nodes. Only extensions that appeared with an
identifier in the valid range in SDP originated by the sender can be
sent.
Example (port numbers, RTP profiles, payload IDs and rtpmaps, etc.
all omitted for brevity):
The offer:
a=extmap:1 URI-toffset
a=extmap:14 URI-obscure
a=extmap:4096 URI-gps-string
a=extmap:4096 URI-gps-binary
a=extmap:4097 URI-frametype
m=video
a=sendrecv
m=audio
a=sendrecv
The answerer is interested in receiving GPS in string format only on
video, but cannot send GPS at all. It is not interested in
transmission offsets on audio, and does not understand the URI-
obscure extension. It therefore moves the extensions from session
level to media level, and adjusts the declarations:
m=video
a=sendrecv
a=extmap:1 URI-toffset
a=extmap:2/recvonly URI-gps-string
a=extmap:3 URI-frametype
m=audio
a=sendrecv
a=extmap:1/sendonly URI-toffset
When using [I-D.ietf-mmusic-sdp-bundle-negotiation] to bundle
multiple m-lines the extmap attribute falls under the special
category of [I-D.ietf-mmusic-sdp-mux-attributes]. All the m-lines in
a bundle group are considered to be part of the same local identifier
(ID) space. If an RTP header extension, i.e. a particular extension
URI and configuration using <extensionattributes>, is offered in
multiple m-lines that are part of the same bundle group it MUST use
the same ID in all of these m-lines. Each m-line in a bundle group
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can include different RTP header extensions allowing for example
audio and video sources to use different sets of RTP header
extensions. It SHALL be assumed that for any RTP header extension,
difference in configuration using any of the <extensionattributes> is
important and need to be preserved to any receiver, thus requiring
assignment of different IDs. Any RTP header extension that does not
match this assumption MUST explicitly provide rules for what are
compatible configurations that can be sent with the same ID. The
directionality of the RTP header extensions in each m-line of the
bundle group are handled as the non-bundled case. This allows for
specifying different directionality for each of the repeated
extension URI in bundled group.
8. BNF Syntax
The syntax definition below uses ABNF according to [RFC5234]. The
syntax element 'URI' is defined in [RFC3986] (only absolute URIs are
permitted here). The syntax element 'extmap' is an attribute as
defined in [RFC4566], i.e., "a=" precedes the extmap definition.
Specific extensionattributes are defined by the specification that
defines a specific extension name; there can be several.
Name: extmap
Value: extmap-value
Syntax:
extmap-value = mapentry SP extensionname
[SP extensionattributes]
mapentry = "extmap:" 1*5DIGIT ["/" direction]
extensionname = URI
extensionattributes = byte-string
direction = "sendonly" / "recvonly" / "sendrecv" / "inactive"
URI = <Defined in RFC 3986>
byte-string = <Defined in RFC 4566>
SP = <Defined in RFC 5234>
DIGIT = <Defined in RFC 5234>
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9. Security Considerations
This document defines only a place to transmit information; the
security implications of each of the extensions must be discussed
with those extensions.
Extensions usage is negotiated using [RFC3264] so integrity
protection and end-to-end authentication MUST be implemented. The
security considerations of [RFC3264] MUST be followed, to prevent,
for example, extension usage blocking.
Header extensions have the same security coverage as the RTP header
itself. When Secure Real-time Transport Protocol (SRTP) [RFC3711] is
used to protect RTP sessions, the RTP payload can be both encrypted
and integrity protected, while the RTP header is either unprotected
or integrity protected. In order to prevent DOS attacks, for
example, by changing the header extension, integrity protection
SHOULD be used. Lower layer security protection like DTLS[RFC6347]
MAY be used. RTP header extensions can carry sensitive information
for which participants in multimedia sessions want confidentiality.
RFC6904 [RFC6904] provides a mechanism, extending the mechanisms of
SRTP, to selectively encrypt RTP header extensions in SRTP.
The RTP application designer needs to consider their security needs,
that includes cipher strength for SRTP packets in general and what
that means for the integrity and confidentiality of the RTP header
extensions. As defined by RFC6904 [RFC6904] the encryption stream
cipher for the header extension is dependent on the chosen SRTP
cipher.
Other security options for securing RTP are discussed in [RFC7201].
10. IANA Considerations
This document updates the IANA consideration to reference this
document and adds a new SDP attribute in section 10.3
Note to IANA : change RFCxxxx to this RFC number and remove the note.
10.1. Identifier Space for IANA to Manage
The mapping from the naming URI form to a reference to a
specification is managed by IANA. Insertion into this registry is
under the requirements of "Expert Review" as defined in [RFC8126].
The IANA will also maintain a server that contains all of the
registered elements in a publicly accessible space.
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Here is the formal declaration to comply with the IETF URN Sub-
namespace specification [RFC3553].
o Registry name: RTP Compact Header Extensions
o Specification: RFC 5285 and RFCs updating RFC 5285.
o Information required:
A. The desired extension naming URI
B. A formal reference to the publicly available specification
C. A short phrase describing the function of the extension
D. Contact information for the organization or person making the
registration
For extensions defined in RFCs, the URI SHOULD be of the form
urn:ietf:params:rtp-hdrext:, and the formal reference is the RFC
number of the RFC documenting the extension.
o Review process: Expert review is REQUIRED. The expert review
SHOULD check the following requirements:
1. that the specification is publicly available;
2. that the extension complies with the requirements of RTP, and
this specification, for header extensions (specifically, that
the header extension can be ignored or discarded without
breaking the RTP layer);
3. that the extension specification is technically consistent (in
itself and with RTP), complete, and comprehensible;
4. that the extension does not duplicate functionality in
existing IETF specifications (including RTP itself), or other
extensions already registered;
5. that the specification contains a security analysis regarding
the content of the header extension;
6. that the extension is generally applicable, for example point-
to-multipoint safe, and the specification correctly describes
limitations if they exist; and
7. that the suggested naming URI form is appropriately chosen and
unique.
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8. That for [I-D.ietf-mmusic-sdp-bundle-negotiation] multiplexed
m-lines, any RTP header extension with difference in
configurations of <extensionattributes> that do not require
assignment of different IDs, MUST explicitly indicate this and
provide rules for what are compatible configurations that can
be sent with the same ID.
o Size and format of entries: a mapping from a naming URI string to
a formal reference to a publicly available specification, with a
descriptive phrase and contact information.
o Initial assignments: none.
10.2. Registration of the SDP extmap Attribute
IANA is requested to update the registration of the extmap SDP
[RFC4566] attribute.
o Contact Name and email address: IETF, contacted via
mmusic@ietf.org, or a successor address designated by IESG
Attribute Name: extmap
o Attribute Syntax: See section 8 of [RFCXXXX].
o Attribute Semantics: The details of appropriate values are given
in [RFC XXXX].
o Usage Level: Media or session level.
o Charset Dependent: No.
o Purpose: defines the mapping from the extension numbers used in
packet headers into extension names.
o O/A Procedures: See section 7 of [RFCXXXX].
o Mux Category: Special.
o Reference: [RFCXXXX]
10.3. Registration of the SDP extmap-allow-mixed Attribute
The IANA is requested to register one new SDP attribute:
o Contact Name and email address: IETF, contacted via
mmusic@ietf.org, or a successor address designated by IESG.
o Attribute Name: extmap-allow-mixed.
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o Attribute Syntax: See section 6 of [RFCXXXX].
o Attribute Semantics: See section 6 of [RFCXXXX].
o Attribute Value: None.
o Usage Level: Media or session level.
o Charset Dependent: no.
o Purpose: Negotiate the use of One and Two bytes in the same RTP
stream.
o O/A Procedures: See section 6 of [RFCXXXX].
o Mux Category: Identical
o Reference: [RFCXXXX]
11. Changes from RFC5285
The major motivation for updating [RFC5285] was to allow having one
byte and two bytes RTP header extensions in the same RTP stream (but
not in the same RTP packet). The support for this case is negotiated
using a new SDP attribute "extmap-allow-mixed" specified in this
document.
The other major change is to update the requirement from the RTP
specification [RFC3550] and [RFC5285] that the header extension "is
designed so that the header extension MAY be ignored". This is
described in section 4.1.
The transmission consideration section (4.1.1) adds more text to
clarify when and how many times to send the RTP header extension to
provide higher probability of delivery
>The security section was expanded
The rest of the changes are editorial.
12. Acknowledgments
Both Brian Link and John Lazzaro provided helpful comments on an
initial draft of this document. Colin Perkins was helpful in
reviewing and dealing with the details. The use of URNs for IETF-
defined extensions was suggested by Jonathan Lennox, and Pete Cordell
was instrumental in improving the padding wording. Dave Oran
provided feedback and text in the review. Mike Dolan contributed the
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two-byte header form. Magnus Westerlund and Tom Taylor were
instrumental in managing the registration text.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508,
DOI 10.17487/RFC2508, February 1999,
<http://www.rfc-editor.org/info/rfc2508>.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, DOI 10.17487/RFC3095,
July 2001, <http://www.rfc-editor.org/info/rfc3095>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
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[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<http://www.rfc-editor.org/info/rfc6904>.
13.2. Informative References
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-38 (work in progress), April 2017.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
IETF URN Sub-namespace for Registered Protocol
Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2003, <http://www.rfc-editor.org/info/rfc3553>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006,
<http://www.rfc-editor.org/info/rfc4585>.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<http://www.rfc-editor.org/info/rfc4588>.
[RFC5109] Li, A., Ed., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, DOI 10.17487/RFC5109, December
2007, <http://www.rfc-editor.org/info/rfc5109>.
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[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<http://www.rfc-editor.org/info/rfc7667>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>.
Authors' Addresses
David Singer
Apple, Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Phone: +1 408 996 1010
Email: singer@apple.com
URI: http://www.apple.com/quicktime
Harikishan Desineni
Qualcomm
10001 Pacific Heights Blvd
San Diego, CA 92121
USA
Phone: +1 858 845 8996
Email: hdesinen@quicinc.com
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Roni Even (editor)
Huawei Technologies
Tel Aviv
Israel
Email: Roni.even@huawei.com
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